Originally Posted by
bubi352
"The maximum speed where full, abrupt control movement can be used without overstressing the airframe" - That definition is completely false! Thanks to the FAA and other flight training books, Va has become a big misunderstanding among pilots.
Va is only valid for positive Gs. Va does not protect the aircraft structure under negative Gs and abrupt sideslip under high dynamic pressure (ie AA587).
I am unsure of the negative portion of Va, but the VG diagram seems to support this claim if you observe the negative G load would be slower than the positive in most cases.
However there are two rules I was taught in aerodynamics that were beat into my head, specifically because at the time we were doing aerobatic training. Va is predicated on two key points during certification, the loading is:
Symmetrical: Meaning level flight, not in turns where one wing may experience a greater load than the other.
Progressive: This one is a little tricky, notice many sources claim "full abrupt" with regard to controls. Now anyone here who has been in an aerobatic aircraft know of "snap speed" or the speed at which you really can go full and abrupt with the controls. Va is determined based on progressive, non snap, meaning not abrupt, loading. A quick pull back, ok, but don't think you can rip that control to your lap right at maneuvering speed and be completely safe, that is why there are snap speeds for aerobatic aircraft. Can anyone verify this with a source other than my flight dynamics professor, he did not source it for us but used our cap 10 snap speed as an example to prove his point.
There is one last item used for determining load factor, that is the material used. Seems like an obvious one so I usually leave this one out.